Distemper in the dog and measles in man, both paramyxovirus infections, may result in demyelinating encephalopathies known as post distemper encephalitis and subacute sclerosing panencephalitis respectively. There is evidence that the onset of demyelination is not exclusively an effect of the virus but of additional mechanism probably allergic or autoallergic in nature. Research in this field was greatly impeded because of the lack of an experimental model in which the factors responsible for the disease could be individually tested and the course of the disease accurately followed. Such a model has now been developed through the isolation of a distemper virus mutant capable of inducing a demyelinating encephalomyelitis in gnotobiotic pups after an incubation time of 5-6 weeks. Experiments have now been designed to test the roles of various factors in the pathogenesis of this disease.